The invention relates to a drag antenna for a sonar system for detecting and/or locating sources of sound in water, and more particularly to a drag antenna of this type that includes at least two side-by-side, laterally spaced-apart, acoustically sensitive lines holding a plurality of electro-acoustical converters (hydrophones) arranged one after another in a direction of the lines wherein the lines are connected at their leading ends with a connecting element that establishes a fixed lateral spacing between the leading ends of the lines.
With a sonar system installed in a towing craft for detecting and locating targets, such drag antennas are used for a more precise determination of the angle of incidence for sound waves radiated by targets in the water or reflected by targets in the water. The at least two parallel-extending, sensitive hydrophone lines or streamers in this case permit a so-called right/left discrimination, meaning a statement as to whether the direction of the sound incidence is on the starboard side or the port side of the drag antenna.
With a drag antenna of this type, known as a twin array (see Joris Jannsen Lok, "17-Ton ALF Heads Dutch Sonar Tests," Jane's Defense Weekly, Mar. 26, 1994, pages 26 to 31), two individual hydrophone streamers are launched separately from the towing craft and are attached at a lateral distance to each other at their leading ends to a towing gear, also called an otter, via a respective pull cable. The towing gear, which in this case accommodates the transmitting section of the so-called active sonar system, is dragged by a towing craft through the water, with the aid of another pull cable. Owing to drag bodies that are attached to the trailing end of the two hydrophone streamers and which have an increased flow resistance, thereby generating a pulling force at the trailing end that is effective in a counter drag direction, the two hydrophone streamers are kept as much as possible in a stretched-out position. Since each hydrophone streamer aligns itself freely in the flow direction behind the front pull cable, a parallel alignment of the two hydrophone streamers, which is advantageous for the right/left discrimination, meaning a constant lateral spacing between the two hydrophone streamers that is sufficient for the line length, is not ensured. Additional and involved methods consequently must be used to detect the exact position of the hydrophones at the moment of sound incidence, so that the exact direction of the sound incidence can be determined.
U.S. Pat. No. 3,414,874 discloses a drag antenna for seismic underwater prospecting in which the leading ends of the drag antenna are connected via a cable (see FIG. 3 thereof). Two otters or drag bodies, arranged at the leading ends of the two outer lines ensure that a lateral force component acts upon the connecting cable during the dragging operation, which lateral force component keeps the connecting cable between the line beginnings stretched tight.